Photographic photometer



Feb. 5 924.

W. SCHLCHTER PHOTOGRAPHIC PHOTOMETER Filed Nov. 20, 1923 Egg Fly-5L.

I C lagj.

Panna-a Feb. s, 1924.

'WILHELM SCHLICHTEB, 0F FREIBURG I. BREISGAU, GERMANY.`

PHTOGRAPHIC PHOTOMETER;

- Application tiled November 20, 1923. Serial No. 875,877.

l T o all whom t may concern:

VBe it known that I, WILHELM SoHmoH- 'rnn,' a citizen of the German Republic, residing at Freiburg I. Breisgau, Germany, have yinvented certain new and useful Improve- `ments in Photographic Photometers, of

. cated a which ythe following is a specification.

My invention relates to improvements in photographic photometers, and more particularly in meters of the type in which the actinic brightness of an object or scenery to be photographed in ascertained b means of checking fields of graded illumination lojacent to an image of the said object or scenery projected throu h a suitable filter, the brightness of the sai picture being made to correspond to that of the checking fields by more or less filtering the ra s by means of an iris stop or the like. T e checking fields are illuminated either b means of a small auxiliary lamp of a stan ard intensity of light, or by means of rays i of day light coming in the direction vfrom the said object or scenery or the space located in front of the meter, and measuring the intensity of the daylight by means of sensitized photometer paper exposed to the lightl andk colored thereby according to a standard and the intensity color. The proper time of the exposure is ascertained from the setting of the irwis stop of the auxiliar lightl used for illuminating the checking elds'or the time exposure 'of the photometric paper, which values are computed together with the sensitiyeness of the negative plate and the opening of the iris stop of the camera by means of a calculating scale. independent of or i. connected with the apparatus. Instruments of this Vtype have been-described in my German Patent No. 350,236, and a brief description will be found in Scientific American of September 1923, page 183.

One o the objects of the improvements is to provide an apparatus f the class referred to by means of which the time of exposure may be determined in an exact wy and taking into consideration specific purposes of the picture and the character of the negative plate. Another object is"to provide an instrument in whichthe illumination of the checking fields ma be graded 'at any time in a correct way. ith these and other objects in View my invention consists in the matters to be described hereinafter and particularly pointed( out -|in the appended claims.

In the "accompanyin drawing,

Fig. 1, shows the ogarithmic gradationil i diagram of a sensitized plate of the market, Figs. 2 to 7 are diagrams illustrating the relation betweenl the diagram shown in Fig.

1 and the scale of the intensities of the parts of the object or scenery,

Fig. 8, is a longitudinal section of the instrument, A

Fig. 9, is a detail View showing the translucent glass designed to have the image projected thereon and showing the checking fields,

y Fig. 10, is a similar view showin the filter placed at the front end of the p otometer for grading the intensity oflight falling on the checking fields, v

Fig.y 11, is an elevation of the instrument, Fig. 12, is a sectional detail-view showing a glass strument, f

Fig. 13, is a side view of Fig. 12, and

Fig. Y14.-, shows a sector used for partly covering the illuminated field produced by the objective glass and filter used in connection with the in-v `used in connection 4with the filter shown in Fig. 12. lEhe operation of a photometer of the type referred to is based on the fact that the ir- 4.

the image of the object or regular values of j ht into. relation `with a scenery are brou single light intensity, for, illuminating thechecking fields. is done by hotometiically balancing the brightness of) the dark, median, and bright portions of the image with the different checking fields. Buttheie arises a difiiculty in so far vas the portions vof different brightness of the image, which may extend over a very broad area of contrast, must be brought into photometric relation to the standard checking fields which include a limited area of contrast. For example, it may be assumed that there are three checking fields, a bright one, a median one, anda dark one, and that the portions of the image are balanced by means o f a shutter by means viz. the intensity used v 'This of which the brightness of the image is made eciial to that of the constant checking fields. urther, the grades of the checking fields and the constant of the iistrument may be such that when making a part of the image equal in brightness to the dark, the

median, and the bright checking fields, ex.- posures are obtained which correspond to the points A, B, and C lof the logarithmic gradation diagram of sensitized plates of the (ffl trade shown in Fig. l,L the area trom A to C being such that the scale oit intensity ci highly contrasting emposures` is covered thereby, and that the said area is mainly represented by the median portion oi 'the gradation da gram, which is substantially rectilinear.

New, it he, first, that the area of contract ot the image scale is equal to that ci the checlringscalmor equal to ihm-C, second, that it is smaller than the said scale, and,

c third, that it is larger. ln the hrst case an unequivocal setting ci the instrument is obtained, and the rectilinear-part oi the gradation diagram is made use oi in the best Way, it the instrument is set in such a Way that the darkest portion ci the image corresponds to the dark checking iield, and the brightest portion oi the image to the brightest checking iield, in which case the scale of the picture covers the part lil-C of the diagram.

lin the second case, which vvill practically.

mainly the lovver part ci the gradation dia' gram, in the position shown in Fig. 3 in which the brightest part of the image is set on the bri htest checking field the image uses mainly tie upper part oi the gradation diagram, and in the position shovvn in Fig. d the scale ci the image is uniformly distributed over the median checkin ield and at opposite sides or the center "il of the rectilinear part of the gradaticn diagram.

till

dll

ln the third case similar diagrams of the relative positions of the scale oi the image and the checking elds are obtained, which diagrams have been illustrated in Figs 5 to 7, the long lines shown below the short ones representing the scale' ot the image, and the short or upper ones that oi the checking fields. ln thus case the normal plate can not l reproduce the Whole scale of the image with a good shading. lin the relative position shown in Fig. 5 the portions ot the image set beyond the point C and so as to be brighter than the bright checking field are within the tield ci being lover-exposed. in the relative position shown in Fig. ti the parts ot the image verh-ich are darker than the dark checking held and y'which are'in viront ot A are in the held ci underexposure, and in the relative position shown in hig. 7, which corresponds to the diagram shown in Fig. d, the image ae a rvhole is uniformly distributed over the linear gradation.

' Experience has shown that when providcheep/ii ing only three checking heide and providing the area included between the bright and dark iields so lar e that also highly contrast ing ima 1es e included in the said area, the condition oi the uniform gradation is not sui'liciently satisfied in the 1Whole area, so that the negatives obtained for example in case oi small contrasts ci the image with the relative positions re resented b hic's. 2 3

and d are not uniform as tar as gradation is concerned. l? or example, very monotonous pictures were frequently over-exposed, when setting the same with the brighest part of the image in position corresponding to the bright checking held, as is shown in Fig. 8, and particularly where the gradation curve of the negative plate begins to become iiat Within the area trom B to C. i

li, however, the area trom fr to C is reduced so liar that, also With plates of re duced gradation, the said area is rectilinear, the photometer tails/to give good results in case oi' highly contrasting pictures, Where an' information las to the con trast area oi the picture and the distribution thereoiE on the rectilinear partis important even for expert photographs.'

llt iollotvs that a universal use oi the instrument is insured onl it the measuring apparatus is constructe so that the position oi the scale oi the image relatively to the checking scale may be more exactly ascertained. p

rTheoretically, this condition might be satised by providing a series ont checking fields between the dark and bright one, instead of a single one, so that the extent and the position of the image scale may be directly read trom the checking scale. However, to provide a lar e number of checking iields is undesira le, because thereby the cost or the instrument is increased and the operation thereof is less simple by reav son or the large number ot checking fields, and, particularly, because an amateur would hardly be able to set the instrument in a proper Way.

"lhe object oit the improvements is to provide ,a photometer of the class referred to in vvhichonly a limited number ot checking helds, say three of them, are provided, and in which the area of the said iieldsis fur- `ther sub-divided by properly graduating the raduation marks of the' meter, so that the instrument is suiiiciently simple in operation to permit the use thereof by amateurs tor ascertaining the time oi exposure, and that the expert is enabled to produce a picture answering specific purposes and to take into regard the character of the sensitized plate. lf the ratio oit the intensities oit' the bright and dark checking fields is d: Z and the logarithmic area of contrast oi the checking table ie ll2=logf- @ll/l (which is .ttin Fig. l), the photometering scale is graduated in such a way that, when further opening the photometer shutter by one graduation markth'e brightlio' within certain limits, a uni orm arrangement of the whole instrument bein obtained, if the checking scale an the photometering scale are based on the same ogarithmic scale unit, which is also made use of in the calculating system connected with the instrument. The latter being ordinarily log. q/, or in a round ligure 1.5, in order to obtain simple numerical relations, also K will be made: K=nlog- 1/5 and accordingl J/i=\/ 2.' Therefore7 for for example, may be 16 or 32 and n ma be 8 or 10.

v mon light dispersin or roughened he photometering scale is gaged by means ofthe checking scale. Therefore, in"

-the construction of the photometer it is important that the Agraduation of the brightness of the checking fields be exactly known and that it is possible at any time to reproduce the same with suflicient accuracy. n my improved instrument an exact graduation is obtained by admitting the light to the surfaces designed .to show the checking elds through chambers having a comfront face provided for example by roug ened glass which face receives the checking light and throws the same through apertures of different areas on the plane of the image and at the side thereof. Thereby I am enabled to produce a graduation which depends on the areas of the apertures so that there is no technical difficulty in producing in the checkin fields the graduation which corresponds to efinite values.

In'order to show the manner of embodying the invention in a photometer I have shown an example in which thetchecking fields are disposed concentrically around the image, and in which the auxiliary light intensity is provided by the light coming from the space in front of the instrument,

which light is lmeasured Jin an absolutely actinic way by means of sensitized photometer xaper connected with the instrument.

In ig. 8 I have illustrated a longitudinal section of theinstrument. As shown in the said figure the instrument comprises a tube` h containing a plate d of translucent glass lglass, and a tube"c of smaller diameter whic extends from the plate d to the front end of the tbe and divides the plate d into two'concentric sections. At the front end of the tube c there is an objective a and at the rear of this objective is an iris stop b, the objective having the function to project a virtual reversed image of the object or scenery on the plate ci, and more particularly on the inner circular portion thereof. At the rear end of the tube h there is an ocular lens e and an actinic filter g consisting for example of cobalt glass through which the image produced on the plate 'al may be inspected, the eye of the photographer being shown at f. The checking light is admitted through the annular chamber provided between the inner tube c and the outer tube h and bounded at.' the front end the said tubes and a glass plate 7c placed thereon and at its rear end by the concentric part of the plate d located between the tubes c and h. The said chamber is divided by four radial partition walls 15 into four sectional chambers of segmental cross-section, Fig. 8 showing only two of the said partition walls, the lines of intersect-ion of the 4 of the instrument by a flange a' connecting e objective glass c is transparent, and the part` l of the flange e' located between the radial lines p and g is cut out, the part bounded by the lines g and r shows holes 16 of comparatively large diameter and the part bounded by theflines. fr and s shows holes of smaller area. Therefore, the annular part-of the disk 7c receives the light from the space in front of the instrument and transmits the lsame through the holes made in the flange c' to the sections or fields enclosed respectively between the lines Land m, m and n, and n and o of the translucent plate d, the amount of the light transmitted to the fields depending on the aggregate area of the holes made in the corresponding parts of the flange Thus the aforesaid checking fields are provided, the relative illumination of which depends on the area of the holes of the flange z'.

The chamber included between the lines s, p, o, and Z is closed at its front end so that -the field included between the lines o and .Z appears black. Therefore, this field does not provide a checking field, and lthe sectional chamber located in front of the same is mainused for receiving the operating pin of l tiie iris stop.

In the example described herein the num ber and size of the holes of the flange z' is such that, when the area of the cut-out portion located between the lines p and is F1,

the aggregate area of the holes 16 is and proceeding from the said setting three scale sections on the scale u, etc.

Further, the area of the contrast of the image, that is the number ofthe scale sections comprised thereby may be'measured. For this purpose the outmost setting -operations must be performed, that is, darkest part of the imagezdarkest field (A), or brightest part of the imager-brightest field (C), and the difference of the values thus obtained as expressed in scale sections must be subtracted from or added to 10.

If, on the other hand the arithmetic middle of the outmost values obtained by the setting operationis taken, I obtain the setting in which the center of the scale of brightness of the image coincides with the median checking field, that is with the point B shown in Fig. 1,` whereby in any case and independentlyof the extent of the contrast of thepimage the most uniform distribution of the scale of the image on the linear gradation is insured. This setting method is particularly important as far as the practical value of the photometer is concerned because it can be performed without any special knowledge or discrimination and by amateurs,` and it gives a time of exposure independent of the image in a purely mechanical way, which time of exposure may be termed the correct time of exposure.

The expert photographer is enabled to lset the scale of the image in any position relatively to the'checking scale and to choose the said position according to any considerations, for example kaccording to the i particular purpose of the exposure or the character of the sensitized plate.

Further, the photometer is constructed so that it is useful for determining the time of exposure in case of indoor photography. In indoor photographv the picture is ordinarily taken from t e side of the window, so that also the image and the check ing fields must be photometrically computed in this direction. Therefore the checking light L falling on the head of the photometer comes from the inner part of the room and it is ordinarily, so dim that the time of the exposure of the photometer paper is too long. In m' improved'photometer instead of the dim ight reflected from the inner parts of the room the light from a window or from any other direction which is much brighter may be used Afor exposing the photometer aper. The ,intensity of the said light may e L and the reduced time of exposure resulting therefrom may be A.

When .setting the image while looking in the direction towards the interior of the room, and using in combination with the value thus obtainedthe time exposure A' which isv much shorter than the correct time of exposure a, the time of exposure would be reduced at the ratio A/A=L/L. Therefore in my improved photometer means are vprovided -for determining the ratio L/L by a subsidiary measurement, and. to compensate the error caused by introducing the value A instead of the correct value A by correction of ythe time of` exposure of the ratio L /L.

The subsidiary apparatus consists of a translucent glass plate 30 adapted to be placed on the head of the photometer and a dark shutter within the field of the image which shutter has for example the form of a black sector e, as is shown in Fig. 14. As appears from Fig. 12/the translucent glass disk 30 is fitted in a -'ring 31 having a lining 32 of velvet by means of which it is placed on the head of the photometer. At -)the partlocated in front of the annular section comprising the checking fields and confined in Fig. 13 between the dotted circular line 33 and the ring 31 the translucent disk. is made opaque. If therefore the translucent disk 30 is put on the head of the photometer the central field of the image appears as a uniformly lig'ht field within the dark checkin fields, the Asector a appearing as a dark bo y within the said light field.

If now, starting from the maximum aperture, theshutter of the photometer is closed the illuminated field gradually loses in brightness, until finally the :sector e disappears. The u and the aperture of the shutter are smaller in the degree as lthe light falling on the. translucent disk is brighter. For example, when makingI the observation in a direction towards the interior of the room, so that the light intensity is-L, the sector will disappear when the shutter has been set to the Value 9. But near the Window it will disappear by reason of the brighter light L1 only when the shutter has been set to the value 3. Therefore in the second case a blinding .corresponding to 6 scale sections or. at the ratio 1,/2a as compared to the first operation is necessary in order to obtain the same degree of darkness of the field of the image. It follows that inthe present example L'/L is equal to 'y/, which means that inthis case thetime of tained on the basismA1 must beincreased at the said ratio. But it is not necessary to determine the coefiicient of the correctiony by calculation. By reason of the correspending arrangement of the scales 'a and fw, from the time of exposure obtained on the scale w with the value A Iproceed a number of scalemarks equal to the difference of the measurements made with the sector z in figure set thereby on the scale exposure ob- I a direction for increasing thetime of expov titl ln the same way l may proceed Whenever the checking light is too Weak so that the photometer paper is not colored Within a reasonable time, and there is a better light coming1 trom another direction. As compared to other methods my improved cor' rection method is preferable, because .the sensitiveness ot the eye. is the same when making the measurements, provided the measurements are made in the same Way and Within a short time one after the other. Therefore the result must be a good one, and it s based on the sensitiveness of the photometer paper. rlhe measurement is hardly less reliable than a measurement made With direct exposure, and the lield ot the use ot the photometer is extended to conditions orD light in which the determination et the time oiE exposure was heretofore particularly ditticult.

ln case ot very bright sceneries the darkening ot' the photometer paper 2l is trequently completed with a short period ot' time, So that it is impossible to get exact results. lt has heretofore been proposed to expose the said paper at the rear ot a lter adapted to weaken the light admitted 'to the paper. ds now practied, by thus weakening toe actinic effect on the paper, the paper is made less visible to the eye, so that it is ditlicult or impossible to compare the color produced on the paper with the standard color. l have 'ioundy that a more usetul `filter is obtained by making the same from a material 1which does not dampen the rays which are not chemically active, but only those rays Which are chemically active. l have `found that the sensitized papers novv in use in photometers are not ailected by light rays oit a Wave length ot more than 500 p. u. @n the other hand, there are dyestutls tor filters in which the curve oit absorption rapidly rises near the said vrai/e lengths and shows high absorption with the whole actinic part of the spectrum, and which are very permeable in the green and yellow part ot the spectrum in which the scnsitiveness ot the eyes is at its nrarrimumo An example ci a dyestuttJ of this character is .the Rapidiiltergelb ot the l-lchster Farbwerlre.

Therefore, my invention consists in using a li'lter tor the photometer provided in my instrument in which the dyestutl and vthe dosing thereot2 are chosen with relation to the sensitized paper ot the photometer so that, With a sufficientliltering etilect increasing the time of exposure, the part oit the spectrum within which the paper is exposed is absorbed in a higher degree than the yellow and green' zones ot the spectrum in 'which the sensitiveness of the eye is at its maximum.

ln the construction shown `in lFig. ll the i eclater thereon which is tor-med with a Window 36 covered by atilter made 'from a gelatine iilm dyed with a suitable dyestutl, preterably the aforesaid ltapidiltergelb ot the Hchster Farbwerlre.

l claim:

l. An instrument tor ascertaining the time ot exposure in photography, comprising a casing, means for determining the actinic value ot the light, means for producing an image of the object or scene to be photographed, and a member Within said casing adjacent the image and adapted to have different shades ot light produced thereon, and a tilter located in position tor being traversed by the rays ot light used for shading said member and t'ormed in the sections corresponding to the. said shades with passages ot diilerent aggregate area tor the non-filtered passage ot the light therethrmigh 2. An instrument ot the class claimed in claim l, in Twhich the scale oit the photometer arrangement by means of which the image and the checking member are balanced is designed on the basis oil the logarithm of the ditlerence of the illumination.

3. An instrument as claimed :in claim 2, f in 'which the logarithmical unit ot the scale ot the photometer is Z/n ot the extent ot contrast included in the computing scale, n being a whole number.

fl. An instrument as claimed in claim 3, in which the logarithmical unit ot the scale corresponds to the logarithmical calculating system ior determining the time oit exposure from the photometric value and the intensity ot the checking light.

o. instrument as claimed yin claim 4l, in which the common unit ot the scale is o. llnstrument as claimed l in claim 2 equipped with a light filter and an obstructing member located inthe iield ot vision and adapted to 'be made to disappear by gradually darkening the lield ot vision.

7. ln combination, with a photometer as' claimed in claim l, the means tor determining the actinic value ot the light comprising a checking color, and means to `expose sensitized paper, ot a lteriadapted to be placed in iront ot the paper and having vhigh absorption Within the part of the spectrum acting on the paper and a low ab sorption within the part ot the spectrum lltl Within which the sensitiveness ot the eye is '5120 at its maximum. I i

ln a photometer as claimed in claim f 7, in which the filter is provided by a yellow tilter oit the character usual in photography. ln testimony Whereot ll adir my signature in presence ot tvvo Witnesses;

WILHELM SClllilCltll'lER. `Witnesses:

ller/ano Temen, lEnrc W. drama. 

